As much as I appreciate the Gonzo work of HST, my favourite book of Hunter is still his first one called Hell's Angels back from the late 60s. It's a very powerful piece of observing journalism and I highly recommend this particular read to anyone interested in books.
Fear and Loathing on the Campaign Trail 72 may be his most important work in terms of political insight. It's almost stunning how on point his criticisms of the relationships between politicians and the media 50 years ago are and continue to ring true. I feel like it's a pretty important book for anyone with interest in American politics.